1. Field of the Invention
This invention relates to a device for neutralizing the access to an integrated-circuit zone to be protected. The invention is particularly well-suited for use in the field of memory cards.
2. Description of the Prior Art
The field of memory cards partly covers the field of credit cards and payment cards. In a memory card, items of information relating to the identity of the card-holder, to the card-holder's secret transaction code as well as to the nature of operations which may be performed with this card are written in memory. In the application concerned, an electronic integrated circuit is employed for the fabrication of this memory and is inserted on the card. In addition, other items of information relating, for example, to the balance of a bank account may also be stored on the card if so desired. In order to guard against forgery, it is necessary to store data relating to identity, secret code, and to performable operations in such a manner as to ensure that they cannot subsequently be modified. This is not the case with a bank balance which can be modified as purchases or bank transfers are being made. Any information which is required to remain inviolable must accordingly be stored in particular zones or areas of the memory and it is thus necessary to neutralize the accesses to these areas once the useful items of information have been written therein.
In the present state of the technique, the zones to be protected are memory areas of integrated circuits. The integrated circuits considered here usually comprise a microprocessor connected to the memory. The introduction of information into the memory area to be protected is obtained by means of a set of connecting leads providing direct access to this area. Series of electrical pulses having the function of introducing the useful items of information into the memory area concerned are applied to the terminals of the integrated circuit to which said leads are connected. In order to prevent subsequent access to this zone, consideration has been given to the possibility of forming a fuse section in series. When the items of information have been entered into the memory, a strong current is passed into the connecting lead, the fuse heats-up and melts, thus simply decoupling the integrated circuit from its access terminals.
The flow of a strong current through the fuse section can obviously not take place by feeding said current into the memory since it would be liable to damage this latter. When performing a fuse-blowout operation, current is injected through a first terminal connected to one end of the fuse. This so-called injection current is shunted to a second terminal via a bypass which is connected to the other end of the fuse, namely the end which actually gives access to the memory. Although access to the memory is no longer allowed through the first terminal, this does not hold true, however, in the case of access to the memory via the second terminal which is connected to the bypass. Thus in the final analysis, no progress has been made in solving the problem since an external operator can still gain access to the memory zone to be protected via the second terminal.